No load sensing device for microwave ovens



' Sept. 8, 1970 Filed Nov; 4, 1968 MICROWAVE OVEN D. B. HAAGENSEN ETAL NO LOAD SENSING DEVICE FOR MICROWAVE ,OVENS POWER MAGNETRON SUPPLY "A" TRIP CIRCUIT 25 29 L POWER MAGNETRON SUPPLY "B" 24 INVENTORS DUANE BHAAGE/VSE/V SAMUEL 6 SWEET airilk y g ATTORNEY United States Patent US. Cl. 21910.55 8 Claims ABSTRACT OF THE DISCLOSURE A no load sensing device is provided for a microwave oven to cut off the magnetrons when a predetermined temperature is reached within the cavity of the oven when no foodstuff has been placed in the oven. The no load sensing device comprises a plurality of rods which are connected to a U-shaped rod. The no load sensing device is mounted inthe cavity of the oven and several of the rods are connected to a housing having a thermostat therein. The rods are arranged and located so as to not interfere with the,,radiation pattern, however, the rods are of such a configuration that they will transfer the heat penetrating the rods to the thermostat so as to give a true indication of the temperature resulting from the quality factor established within the cavity.

FIELD OF THE INVENTION This invention relates to microwave ovens and more particularly to a no loading sensing device for such ovens.

DESCRIPTION OF THE PRIOR-ART It is well known that electromagnetic Wave energy may be utilized for heating foodstuff or other lossy dielectric materials. The foodstufi or other materials are placed in a cavity of a microwave oven and are exposed to electromagnetic wave energy that is supplied by a suitable source, e.g., a magnetron. After a relatively short period of time during which the foodstuff is subjected to the electromagnetic wave energy, heat will be generated within the foodstuff to accomplish the desired cooking of the foodstuff. However, if the microwave oven is turned on when no foodstuff is within the oven, the heat generated by the electromagnetic wave energy must be absorbed by other materials within the cavity of the microwave oven. In most cases, the other materials within the oven are not lossy dielectric materials. Thus, the electromagnetic wave energy will not be absorbed within this cavity, but will be absorbed by materials comprising the door seal of the microwave oven, or the energy will be reflected back to the magnetron. This reflected energy at the magnetron will eventually increase the temperature at the magnetron to the point where considerable damage to the magnetron will occur. Since the magnetron is the single most expensive item within a microwave oven, continued replacement of the magnetron is highly undesirable.

In the past, a thermostat has been placed in proximity of the anode of the magnetron to detect temperature at the magnetron. Such thermostats were set to cut off the magnetron just before a temperature was reached which would be deleterious to the magnetron. However, considerable damage to the'cathode of the magnetron may be experienced before such thermostats cut off the magnetron. In addition, considerable damage to materials comprising the door seal or the shelf for supporting the foodstuff might occur before such thermostats cut off the magnetron because of the phase relationship of the reflected energy at the anode of the magnetron. It is quite likely that only a small percentage of the electromagnetic wave energy within the oven is reflected back to the magnetron. In such 3,527,915 Patented Sept. 8, 1970 cases, the thermostat would not react until damage to the door seal or the shelf of the microwave oven would occur. Therefore, prior sensing devices were not so located as to be sensitive enough to react quickly in cutting off the magnetron.

To ensure that the magnetron is cutoff before any damage occurs, a no load sensing device must be placed within the cavity of the microwave oven. Such a device would detect the temperature in the cavity of the oven. The temperature in the cavity bears a direct relationship to the quality factor of the cavity. The quality factor of the cavity is the energy stored in the cavity divided by the energy dissipated in the cavity.

Accordingly, it is an object of the present invention to provide a novel means of protecting against damage to microwave ovens.

. It is a further object of the present invention to provide a no load sensing means in the cavity of a microwave oven to protect against damage to the magnetron and other materials within the microwave oven.

It is a still further object of the present invention to provide a no load sensing device which cuts off the magnetron upon detecting the temperature resulting upon reaching a predetermined maximum quality factor of the microwave oven.

SUMMARY OF THE INVENTION In accordance with the objects set forth above, the present invention provides a no load sensing device which cuts off the magnetron, or magnetrons, upon detecting the temperature resulting upon reaching a predetermined maximum quality factor of the microwave oven. The no load sensing device is placed within the cavity of the microwave oven and comprises a plurality of rods, which may be of a particular configuration and a particular size depending upon the material utilized in the rods. The rods are connected together, and housing for a thermostat is attached to several of the rods to provide a path for the heat penetrating the rods to efliciently travel to the thermostat. The rods are so arranged as not to interfere with the radiation pattern established within the oven, but are also arranged so as to absorb suflicient heat which is a true indication of the temperature resulting from the quality factor established within the cavity.

BRIEF DESCRIPTION OF THE DRAWINGS Additional objects, advantages, and characteristic features of the present invention will become readily apparent from the following detailed description of the preferred embodiments of the invention when taken in conjunction with the accompanying drawings in which: 7 FIG. 1 is a side elevational view of a microwave oven, with a portion of the side housing cut away to expose the cavity of the microwave oven, in accordance with the present invention;

FIG. 2 is a perspective view of the deep Well cavity and no load sensing device in accordance with the present invention; and

FIG. 3 is a simplified block diagram of no load sensing device connected to the power source in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a side eletional view of a microwave oven 10 in accordance with the I present invention. On the right side of FIG. 1, a door 11 along with a handle 12 is shown to facilitate the insertion and the removal of foodstuff to and from a cavity 15 of the microwave oven 10. The numeral 13 designates the controls for operating the microwave oven 10, while the numeral 14 designates the area occupied by the control circuits, the power supplies, the magnetrons, etc. which are utilized to operate the microwave oven 10.

FIG. 1 is shown with a portion of the side housing cut away to expose the cavity 15 within the microwave oven 10. In the process of cooking foodstuff, the items to be cooked would be placed onto a shelf 16, which may be of any suitable dielectric material, for example, pyro-ceram, which is a highly heat resistant Pyrex. Such a material does not interfere with the radiation pattern and will not crack under normal microwave oven operating conditions. Shown below the shelf 16 is another portion of the cavity 15, and this portion is designated as 15a. Such cavity portions are well known as the deep well cavity portions of microwave oven cavities and are utilized to provide a more evenly distributed radiation pattern in order to provide for more efiicient overall cooking of foodstuffs. Located within the deep well cavity 15a is a no load sensing device 17.

The no load sensing device 17 is utilized to detect the temperature in the cavity 15 of the microwave oven 10. The temperature in the cavity 15 bears a direct relationship to the quality factor of the cooking cavity of a microwave oven, i.e., the quality factor of the cavity 15 is equal to the energy stored in the cavity divided by the energy dissipated in the cavity (Q energy stored/energy dissipated). Thus, if no foodstuff is present in the microwave oven to absorb the electromagnetic wave energy, the Q increases with the result that the temperature increases. In practice, it has been found that when operating the microwave oven 10 at 2,000 Watts, the no load sensing device 17 has cut off the magetrons in three minutes with no foodstuff in the cavity 15, without incurring any damage to the magnetrons or other materials in the oven.

Referring now to FIG. 2, there is shown a perspective view of the no load sensing device 17 as it is located within the deep well cavity a. The shelf 16 has been cut away to expose the no load sensing device 17. The no load sensing device 17 is shown placed approximately twothirds of the distance down in the deep well cavity 15a, and is further placed equal distances from the cavity walls, so as to ensure that it will not interfere with the radiation pattern and to further ensure that no arcing occurs between the no load sensing device 17 and the walls or the bottom of the deep well cavity 15a. The no load sensing device 17 comprises a U-shaped rod 19 having a plurality of parallel pairs of smaller diameter rods 20, equally spaced, and permanently attached thereto. The rods may be constructed of any suitable material which would have the necessary characteristics to allow sufficient penetration of the heat generated by the electromagnetic wave energy within the cavity 15. The penetration and transfer characteristics would have to be such that proper indication of the temperature in the cavity 15 would be detected. The aforementioned characteristics are thoroughly taught in a discussion of skin effect, by Arthur R. Von Hippel, entitled Dielectrics and Waves, John Wiley and Sons, Inc., New York, 1965, pages 6 1 and 62.

In a preferred configuration, the U-shaped rod 19 is of one quarter inch diameter and the rods 20 are of oneeighth inch diameter. Six rows of parallel pairs of rods are shown. The rods 19 and 20 of constructed of stainless steel. This configuration proved to have very little, if any, effect upon the radiation pattern within the cavity 15. The configuration of stainless steel rods in accordance with the present invention provided the necessary penetration of heat and the necessary transfer of heat throughout the no load sensing device 17. In addition, the use of stainless steel allows the rods to heat quickly and evenly and further allows the rods to cool quickly, which is an important consideration in order to obtain maximum use of the microwave oven. Permanently attached to several of the rods 20 is a housing 18, preferably constructed of stainless steel, having a hollow portion in which a thermostat 21 is located. In practice, the housing 18 has been a cylinder to facilitate the insertion of the thermostat 21 which is encased within a cylinder-shaped protective coating. The housing 18 may be attached to the rods 20 by any suitable means, for example, by soldering. The no load sensing device 17 is rigidly afiixed within the cavity 15a by locking nuts 22 and 23. Leads 21a and 21b are shown connected to the thermostat 21 and these leads may be properly connected within a control circuit that is shown in FIG. 3. It is readily apparent to those skilled in the art, that the no load sensing device 17 may be located anywhere within the cavity 15 so long as it does not interfere with the radiation pattern. If the no load sensing device 17 'were located in some visible area in the cavity 15, it could be porcelainized to facilitate cleaning. In such a case, consideration would have to be given to the absorption characteristics of the porcelain coating in determining the cut off temperature of the thermostat.

Referring now to FIG. 3, there is shown part of circuits designated by the numeral 14 of FIG. 1. The thermostat 21 is connected to a pair of leads 21a and 21b. The lead 21b is connected to a ground 25 through a relay 31, while the lead 21a is connected to a trip circuit 24, which may be a relay. The respective outputs of the trip circuit 24 control a pair of power supplies 27 and 26, which in turn control a pair of magnetrons 28 and 29, which are designated magnetron A and magnetron B, respectively.

As is shown in FIG. 3, there are two magnetrons 28 and 29 employed in the microwave oven 10. When the microwave oven 10 is at full power, both magnetrons will be operating. In that case, a certain field strength will be present in the cavity 15 and the thermostat 21 will be connected in this circuit through the relay 31. The thermostat 21 is designed to open at approximately 180 F.; therefore, when the thermostat 21 detects a temperature of 180 F., the ground 25 for the trip circuit 24 -will be disconnected and power will no longer be applied by the power supplies 26 and 27 to the magnetrons 29, and 28, respectively.

On the other hand, if the microwave oven 10 is at halfpower, only one of the magnetrons is operating. In that case, a field strength will be present in the cavity which will be less than the field strength established in the case when two magnetrons are operating. When operating at half-power, a thermostat 30 will be connected in the circuit through the relay 31 via leads 30a and 30b. The thermostat 30 is designed to open at approximately F.; therefore, when the thermostat 30 detects a temperature of 150 F., the ground for the trip circuit 24 will be disconnected and will no longer be applied by the one of the power supplies to its respective magnetron. Of course, it can be readily appreciated by those skilled in the art, that the housing 18 could be adapted to retain both the thermostat 21 and the thermostat 30.

Thus, although the present invention has been shown and described with preference to particular embodiments, for example, a no load sensing device having a particular configuration of rods, nevertheless, various changes and modifications obvious to a person skilled in the art to which the invention pertains, for example, no load sensing means having square rods and being of different size and of different material are deemed to lie within the spirit, scope, and contemplation of the invention as set forth in the appended claims.

What is claimed is:

1. Microwave oven apparatus comprising:

a heating cavity;

means for supplying electromagnetic wave energy to said heating cavity; and

no load sensing means, located within said heating cavity and coupled to said means for supplying said electromagnetic wave energy, for cutting off said means for supplying electromagnetic wave energy at a predetermined temperature which bears a direct relationship to the quality factor established in said heating cavity.

2. Microwave oven apparatus as recited in claim 1 wherein said no load sensing means comprises:

a U-shaped rod; a plurality of pairs of parallel rods connected to said U-shaped rod;

housing means, having an opening therein, connected to a plurality of said pairs of parallel rods for transferring a temperature rise from said plurality of pairs of parallel rods and said U-shaped rod to said housing means;

sensing means located within said opening of said housing means for receiving a temperature rise from said housing means; and

mounting means for mounting said no load sensing means within said cavity.

3. Microwave oven apparatus as recited in claim 2 wherein said heating cavity comprises:

upper portion means for heating foodstuff placed therelower portion means for housing said no load sensing means; and

shelf means for separating said upper portion means from said lower portion means and said shelf means for supporting said foodstuff placed in said upper portion means.

4. Microwave oven apparatus as recited as claim 2 wherein said no load sensing means is mounted in a visible area within said cavity and said no load sensing means is coated with porcelain.

5. Microwave oven apparatus as recited in claim 2 wherein said U-shaped rod is aco-existing triple U-shaped member of stainless steel of one-quarter inch diameter and said plurality of pairs of parallel rods are of stainless steel of one-eighth inch diameter.

6. Microwave oven apparatus as recited in claim 1 wherein said no load sensing means is comprised of a plurality of members constructed of material and so arranged that said no load sensing means provides the necessary penetration of electromagnetic wave energy and the necessary transfer of heat throughout said no load sensing means so that a true indication of the quality factor established in said cavity may be detected.

7. Microwave oven apparatus as recited in claim 6 wherein said plurality of members are constructed of stainless steel and are so arranged that substantially no interference with the radiation pattern occurs, said p111- rality of members are further arranged to prevent arcing of the electromagnetic wave energy between said no load sensing means and said cavity.

8. Microwave oven apparatus as recited in claim 1 wherein:

said means for supplying electromagnetic wave energy includes two magnetrons, and control circuits for selectively operating said two magnetrons simultaneously, or either one of said magnetrons individually;

, said no load sensing means includes a first thermostat and a second thermostat; and

trip circuit means cooperating with said means for supplying electromagnetic Wave energy and said no load sensing means, said trip circuit means for selectively cutting off power to said magnetrons simultaneously, or either one of said magnetrons individually.

References Cited UNITED STATES PATENTS 3,185,809 5/1965 Bohm et al 21910.55 3,281,568 10/1966 Haagensen 21910.55 3,467,804 9/1969 Smith 219-1055 r JOSEPH V. TRUHE, Primary Examiner.

L. H. BENDER, Assistant Examiner 

